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None of it rings a bell for her. Christine has lost 20 years, and every night when she dozes off she loses that day’s memories as well.

A phone call promises help, a clue. Look in your closet, the voice of a man calling himself a doctor tells her. Look for the shoebox with the digital camera in it. Her video diary is there. Dr. Nasch (Mark Strong) is the one who got her to start keeping one.

But something unsettles her, the bits of her past that the doctor, who insists she keep their relationship a secret, tells her. And she’s not sure what to make of the omissions her husband is leaving out of that story “to protect you.”

“So you edit my life?”

“Before I Go to Sleep” hangs on Kidman’s intimate performance. She whispers, girlishly, shocked at being told she had an affair, puzzled that the two men give her differing versions of how she lost her memory. At the beginning of each day, she is passive, naive and trusting. She gets into the car of the man who calls himself her doctor without question.

But as the days progress and the story advances, she adds to that diary and becomes assertive, questioning and suspicious. Some days, she suspects the husband of manipulating her. Some days, the doctor. Some evenings she’s drawn to the man who says he’s trying to heal her, and some she has sex with the man who insists he’s withholding details to save her pain and heartache.

Writer-director Rowan Joffe (he wrote the Clooney hitman thriller, “The American”), adapting an S.J. Watson novel, maintains the mystery at the heart of this puzzle picture and jolts us with the odd shock – a violent flashback, a loud horn blast from a passing truck that nearly hits someone.

But he wisely lets this be an actor’s picture. Strong, often cast as villains, is poker-faced here, close-ups capturing wheels turning that could be a doctor reasoning out a talking cure or someone with reason to keep Christine in the dark.

Firth, most often a romantic lead, wears a deflated look of loss that either masks the grief of a man whose great love has lost her sense of identity or something cagier.

And Kidman lets us feel Christine’s confusion, her desire to not stay in the dark even if every memory retrieved threatens more pain.

Whatever twists this puzzle tosses at us, the film reminds us that a great actor, in close-up, telling a story with just her or his eyes, is still the greatest special effect the movies have to offer. This cast telling this story ensures us that nobody will be dozing off “Before I Go to Sleep.”